It is, of course, necessary to load high power transmitters from time to time under controlled conditions for tuning adjustment. This must be at full, or nearly full output power, and this may take a substantial amount of time. During this time, the load should be as close as possible to the required resistance for precise tuning. This requires the use of a resistive dummy load that can accomodate the full output of the transmitter without random atmospheric discontinuities.
The standard dummy loads must be non-inductive, for obvious reasons, and can be of carbon, or other resistive, non-inductive or capacitive material. However, as power is applied to the resistive dummy load, it must heat up, and, with higher-powered transmitters, must be capable of carrying very-high wattages, and considerable heat, for extended periods of time.
This requires heavier duty resistors--of silicon carbide, or the like--that are physically stronger, and can be made in the larger sizes that will be needed for the heavier loads. Even these must be limited by temperature rise, and must be cooled by some means, such as fans, at least, to keep their temperatures at a safe working level, and prevent burnout. As more wattage is applied from the transmitter, more heat must be absorbed, and larger units, or more and more units will be required, connected in series or parallel to dissipate the increasing heat loads. The basic necessity is to carry away the heat generated by the high powered transmitter across the dummy load.
However, the inevitable temperature rise in a dummy load further complicates the problem by the fact that the resistance of the resistors varies with temperature, and a constant temperature and resistance can only be achieved with predictable resistors, cooling fan velocities, and radio frequency power outputs. Any change in any of these factors, or even the ambient temperature, will change the dummy load temperature and resistance, and reduce the accuracy of the tuning process.
Even under stable conditions, precise temperatures can only be maintained under a constant transmitter output, and separate systems would probably be necessary for different transmitters and outputs. The cooling may be varied to some extent, but can only maintain constant temperatures in the resistors over a narrow range of inputs. It would be almost impossible for cooling fans to maintain constant temperatures of resistors over the extremely-wide range of transmitter outputs.
Aside from the almost-imposibility of maintaining a constant temperature over a variety of transmitter outputs, the resistors for a dummy load for a high-powered transmitter would be bulky and heavy, and take up a considerable space for effective heat dissipation. The fans or blowers to cool the resistors would be bulkier and take up even more space.
Water cooling, instead of air cooling, would be ideal, since it hase a much-higher cooling capacity. However, the water must be insulated from the electrical circuitry to avoid short circuits or changes in resistance. This requires a highly-conductive insulation between the resistive material and the water or liquid that must carry away the heat. This would be analogous to automobile engines, where the heat from the combustion chambers must be transmitted through the engine block to the coolant in a relatively non-uniformand inefficient manner. The radiator, on the other hand can be quite efficient. Water-cooled resistors can provide a more compact and efficient dummy load, but they still lack the ability to maintain constant temperatures and resistance over a wide range of transmitter outputs.
It is therefore an object of this invention to provide a dummy load that will maintain its terminal resistance, precisely, under all load conditions, almost indefinitely.
It is a further object of this invention to provide a dummy load that, as a small, compact unit, can absorb a higher wattage of radio frequency energy than banks of conventional resistors, many-more times its size and weight.
It is a further object of this invention to provide a dummy load wherein the resistive load is in liquid form, and the transmitter output load is applied directly to the liquid that can be circulated and temperature-controlled outside of the dummy load, in a conventional manner, to maintain a constant temperature and resistance within the dummy load at all times and under all output loads.
It is a further object of this invention to provide a dummy load that, along with being much smaller and more compact, is very much cheaper to install and operate.
These and other objects are accomplished by the following dummy load system.